So my boss bought everyone in our web developer's group an Apple Mighty Mouse today. An aptly named product, I must say! The scroll ball works great - even better than a scroll wheel - and the scroll ball button and squeeze buttons have already given me a productivity boost.
My personal setup is to have the programmable buttons activate Exposé, so that the scroll ball button hides everything but the desktop, and the squeeze buttons show all open windows. Makes Exposé much more useful, because you don't have to go hunting for function keys.